Distributed Synchronization

Distributed Synchronization
In single CPU systems Semaphores and monitors Essentially shared memory solutions How about distributed synchronization? Relevant information is scattered Processes make decisions based on local information A single point of failure in a system should be avoided No common clock or other precise global time source exists 4/16/2019 ICSS741 - Time and Coordination

ICSS741 - Time and Coordination
What Is Needed In order to coordinate events in a distributed system We may need to know the time at which a particular event took place We may need to determine the order in which two events took place, or should take place, without respect to the time they actually occur Synchronization requires either Global time Global ordering 4/16/2019 ICSS741 - Time and Coordination

Time Synchronization Is it possible to synchronize all clocks to produce a single, unambiguous time standard? Time synchronization need not be absolute What usually matters is that processes agree on the order in which events occur (not necessarily the time at which they occur) 4/16/2019 ICSS741 - Time and Coordination

Basically two problems with time External synchronization Synchronize clocks with an authoritative external source of time Internal synchronization The internal consistency of the clocks is what matters, not whether they are close to the real time 4/16/2019 ICSS741 - Time and Coordination

Astronomical Time Since the 17th century time has been measured astronomically The event of the sun reaching the highest point in the sky is called the transit of the sun The interval between two consecutive transits of the sun is called a solar day In the 1940s, it was established that the earth’s rotation is not constant The earth is spinning slower 300 million years ago there were about 400 days per year 4/16/2019 ICSS741 - Time and Coordination

Atomic Time The atomic clock was invented in 1948 One second is the time it takes the cesium 133 atom to make 9,192,631,770 transitions Currently about 50 cesium-133 clocks exist Periodically they are averaged to produce international atomic time (TAI) The Bureau International de l’Heure (BIH) maintains the official clock 4/16/2019 ICSS741 - Time and Coordination

Leap Seconds Currently about 86,400 TAI seconds is about 3msec less than a mean solar day Not a problem until noon becomes 6am BIH solves the problem by inserting leap seconds to compensate for the difference Leap seconds are added whenever the discrepancy grows to 800 msec Power companies will increase their frequencies to compensate UTC (Universal coordinated time) is the result 4/16/2019 ICSS741 - Time and Coordination

Obtaining Accurate Time
UTC is an international standard for the current time WWV shortwave radio from Fort Collins (accuracy 0.1 – 10 milliseconds) GEOS satellites (0.1 milliseconds) GPS satellites (1 millisecond) 4/16/2019 ICSS741 - Time and Coordination

Physical Clocks Computer each contain their own physical clocks Timer might be a better word… Utilize crystal that oscillate at a known frequency A count of the oscillations is maintained Software typically takes this count, divides it down, and stores it as a number in a register Most systems provide date/time from the counter Ordering events, in a single machine, with such a clock is easy Provided the clock resolution is fine enough 4/16/2019 ICSS741 - Time and Coordination

Clock Drift Crystal-based clocks are subject to drifting the change in the offset between the clock and a nominal perfect reference clock per unit of time measured by the reference clock Typical drift rates Quartz crystals – 10-6 (about a difference of one second every 1,000,000 seconds or 11.6 days) Atomic clocks – 10-13 4/16/2019 ICSS741 - Time and Coordination

External Synchronization
Lets say you have access to a UTC time source Assume the machine has a timer that causes an interrupt H times a second Current clock value is C When UTC time is t, the value of the clock on machine p is Cp(t) Ideally Cp(t)=t for all p and t (dC/dt should be 1) 4/16/2019 ICSS741 - Time and Coordination

Maximum Drift Rate dC/dt > 1 dC/dt = 1 Fast clock Perfect clock dC/dt < 1 Slow clock 4/16/2019 ICSS741 - Time and Coordination

Synchronizing Physical Time
What exactly does it mean to synchronize two clocks? Clocks inherently suffer from drifting Assuming clocks can always be precisely synchronized in unrealistic Define an acceptable range for the difference in time reported by two clocks (clock skew) A distributed physical clock synchronization service defines, and maintains, a maximum skew throughout the system. 4/16/2019 ICSS741 - Time and Coordination

The Basic Algorithm A wants to read B’s clock A sends a request to B B records its current clock value The clock value is sent back to A B’s clock value is adjusted to reflect travel time B’s clock value can now be compared to A’s Step 4 is difficult to implement accurately 4/16/2019 ICSS741 - Time and Coordination

Interesting Question So you have to adjust your time Your clock is slow – move it ahead Your clock is fast – move it back? Implementations Slow down your clock so it will continually move towards the real time Speed up your clock so it will move towards the real time Just move your clock ahead to the real time 4/16/2019 ICSS741 - Time and Coordination

Cristian’s Algorithm One machine knows the true time Periodically each machine sends a request for the current time T0 Request Time I, interrupt handling time CUTC T1 Measured with the same clock 4/16/2019 ICSS741 - Time and Coordination

Transit Time Estimating propagation time ( T1 – T0 ) / 2 ( T1 – T0 – I ) / 2 If minimum possible propagation delay is known, the estimate can be made better Accuracy can be improved by taking several measurements Any measurement in which T1 – T0 exceeds a threshold is discarded (congestion) 4/16/2019 ICSS741 - Time and Coordination

ICMP Timestamp Request/Reply
Type (17 or 18) Code (0) Checksum Identifier Sequence Number 32-bit originate timestamp 32-bit receive timestamp 32-bit transmit timestamp Same clock so difference is accurate rtt 4/16/2019 ICSS741 - Time and Coordination

The Berkeley Algorithm
Time server is active, and polls each machine periodically for its time Based on the answers, an average time is computed A fault-tolerant average is used Machines are then told to slow down, or speed up their clocks Suitable for systems where no UTC source is available 4/16/2019 ICSS741 - Time and Coordination

Berkeley Algorithm 740 740 Current Time = 720 Move clock forward 7 Current Time = 740 Adjusted TimeA = 730 Adjusted TimeB = 742 Average = 737 Current Time = 737 720 737 +7 Network delay = 10 Network delay = 5 4/16/2019 ICSS741 - Time and Coordination

Network Time Protocol NTP is used to synchronize the time of a computer client to another server or reference time source Client accuracies are typically within a millisecond on LANs and up to a few tens of milliseconds on WANs NTP configurations utilize multiple redundant servers and diverse network paths in order to achieve high accuracy and reliability Configurations can use authentication to prevent accidental or malicious protocol attacks 4/16/2019 ICSS741 - Time and Coordination

NTP Strata Primary Servers (stratum 1 ) sync to UTC source
Secondary Servers (stratum 2 ) sync to primary servers Workstations 4/16/2019 ICSS741 - Time and Coordination

USNA NTP Time Servers 4/16/2019 ICSS741 - Time and Coordination

Rules of Engagement Clients should avoid using the primary servers whenever possible In most cases the accuracy of the NTP secondary (stratum 2) servers is only slightly degraded relative to the primary servers As a group, the secondary servers may be just as reliable 4/16/2019 ICSS741 - Time and Coordination

When to Use a Primary As a general rule The secondary server provides synchronization to a sizable population of other servers and clients The server operates with at least two and preferably three other secondary servers in a common synchronization subnet The administration(s) that operates these servers coordinates other servers within the region, in order to reduce the resources required outside that region. In order to ensure reliability, clients should spread their use over many different servers 4/16/2019 ICSS741 - Time and Coordination

NTP Servers (home page for NTP) List of Primary Servers (100) List of Secondary Servers (110) Our server timehost.cs.rit.edu 4/16/2019 ICSS741 - Time and Coordination

Synchronization Modes
Servers synchronize in one of three modes Multicast Used on high speed LANs Servers periodically broadcast their time Low accuracies, but efficient Procedure-call Similar to the operation of Cristian’s algorithm Symmetric Used by master servers Pairs of servers exchange information Timing data is retained in order to improve accuracy 4/16/2019 ICSS741 - Time and Coordination

NTP Design Goals The four primary design goals of NTP are Allow accurate UTC synchronization Enable survival despite significant losses of connectivity Allow frequent resynchronization Protect against malicious or accidental interference 4/16/2019 ICSS741 - Time and Coordination

Accurate Synchronization
NTP provides the following information relative to the primary server Clock offset Difference between the two clocks Round-trip delay Total transmission time for the messages Dispersion Offsets are predicted Dispersion is a measure of how much the prediction differs from what what reported Large dispersion values indicate inaccuracy 4/16/2019 ICSS741 - Time and Coordination

Logical Clocks Since physical clocks cannot be perfectly synchronized across a distributed system Physical time cannot be used to determine the order in which events occur Logical clocks can be used to order events within a distributed system The essence of a logical clock is the happens-before relationship 4/16/2019 ICSS741 - Time and Coordination

Happens-Before The happens-before relationship is denoted a  b If a and b are events in the same process, and a occurs before b, then a  b If a is the event of a message being sent by one process, and b is the event of the message being received by another process, then a  b If a  b, and b  c, then a  c Any two events that are not in a happen-before relationship are concurrent 4/16/2019 ICSS741 - Time and Coordination

Events 4/16/2019 ICSS741 - Time and Coordination

Lamport’s Logical Clock
To obtain logical ordering, timestamps that are independent of physical clocks are used Lamport clocks follow these rules Each process increments it clock between every two consecutive events If a sends a message to b, the message includes T(a). Upon receipt, b sets its clock to the greater of T(a)+1 and the current clock 4/16/2019 ICSS741 - Time and Coordination

Lamport’s Algorithm 1 2 3 4 5 6 7 8 9 10 8 16 24 32 40 48 56 64 72 80 10 20 30 40 50 60 70 80 90 100 1 2 3 4 5 6 7 8 70 71 8 16 24 32 40 48 61 69 77 85 10 20 30 40 50 60 70 80 90 100 a a b b c c d d e e f f A(6)  F(10)  B(24)  C(50)  D(60)  E(64) A(6) B(24)  C(50)  D(60)  E(64)  F(71) 4/16/2019 ICSS741 - Time and Coordination

Partial Ordering If a  b then L(a) < L(b) Note that L(d) < L(e) Does not imply that d  e Since d and e might be concurrent Plus L(a) might equal L(b) 4/16/2019 ICSS741 - Time and Coordination

Example 4/16/2019 ICSS741 - Time and Coordination

Total Ordering Between every event the clock must tick at least once Since events cannot happen at the same time, attach the process number to the low-order end of the time, separated by a decimal point Now If a happens before b in the same process, C(a) < C(b) If a and b represent the sending an receiving of a message, C(a) < C(b) For all events a and b, C(a) is not equal to C(b) 4/16/2019 ICSS741 - Time and Coordination

Vector Clocks Vector clocks were designed to overcome the shortcomings of Lamport’s clocks A vector clock is an array of times The rules: Initially, Vi[j]=0, for i,j = 1,2 …, N Just before pi timestamps an event, it increments Vi[i] pi includes the value t = Vi in every message it sends When pi receives a timestamp in as message, it takes the component-wise maximum of the two vector timestamps 4/16/2019 ICSS741 - Time and Coordination

Example 4/16/2019 ICSS741 - Time and Coordination

Comparing Timestamps Vector timestamps are compared as follows V = V’ iff V[j] = V’[j] for j=1,2,…,N V <= V’ iff V[j] <= V’[j] for j=1,2…,N V < V’ iff V<=V’ and V != V’ So what? If V(e) < V(e’) then ee’ c and e are concurrent since neither V(c) <= V(e) nor V(e)<=V(c) 4/16/2019 ICSS741 - Time and Coordination

Distributed Synchronization

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